Particle-laden fluids, particularly those containing proteins, fats, cells and cell fragments of the sort encountered in pharmaceutical and biotechnical applications, are filtered according to the cross-flow principle on both laboratory and industrial scales. The goal of the filtration may include the recovery of valuable material, the concentration of substances, the separation of undesirable components and sterilization.
Although cross-flow filtration may be carried out with a single filter cassette, it is generally conducted with multiple filter cassettes in a stacked configuration. Such cross-flow filter cassettes are disclosed in DE 3 441 249 and U.S. Pat. No. 4,715,955. Such filter cassettes are constructed of multiple filter cells comprising stacked flat sections of at least one retentate spacer forming a feed flow passage or slit, a first microporous membrane, a permeate spacer forming a permeate collection passage or slit and a second microporous membrane. Such stacked flat sections are bound to the filter cassette by sealant. The flat sections are provided with aligned openings in their peripheral area that form flow conduits running perpendicular to the surfaces of the flat sections and that are useable as a fluid feed inlet and as outlets for retentate and permeate. Each opening of the spacers which is desired to be closed to the flow conduits is surrounded by fluid-tight sealant in its periphery which extends slightly beyond the opening. The remaining openings in the spacers which are to be in fluid communication with the conduits have no sealant, but are left open.
In cross-flow filtration the fluid feed is pumped through a leading plate and corresponding conduits are in fluid communication with at least one feed flow passage or slit of the filter cassette, causing the feed to flow over the microporous membrane's surface and to be removed as retentate. A portion of the feed permeates through the membrane and is collected in a permeate collection passage or slit and, by means of appropriate conduits and an outlet, is removed from the cassette as filtrate or permeate.
To protect the membrane from mechanical damage which can occur in the transition areas of the sealant and usually caused by excessive pressure on the membrane, DE 34 41 249 suggests the insertion of protective frames between the sections of the filter cassette. In addition to achieving protection of the membranes, the feed flow passage or slit is widened, allowing thicker fabrics to be employed to increase the height of the flow passage. Cassettes provided with such wide passages tend not to be subject to early blockages. However, a disadvantage of such wide passage cassettes is that particles and proteins in the filtered fluids are captured in the matrices of the fabric in the feed flow passage and tend to agglomerate there. This agglomerated material causes a flow restriction of the fluid feed. Known cross-flow filter cassettes having wide passages are therefore not suitable for cross-flow filtration of particle-laden, protein-carrying fluids, because a massive reduction of the retentate often occurs and after a short filtration period filtration is terminated altogether by the buildup of particles, proteins and the like in the feed flow passages.
Thus, the goal of the invention is to provide cross-flow filter cassettes which are appropriate for cross-flow filtration of fluids containing particles such as proteins, fats, cells and cell fragments such as are encountered in pharmaceutical and biotechnological applications, and which are characterized by a long filtration life and a high throughput of retentate and filtrate flows.